It has recently been discovered that the strength of an adhesive bond between two substrates, for a given quantity of a selected hot melt adhesive, can be substantially improved if the adhesive is used as a cellular foam, rather than in the conventional way as a molten but unfoamed adhesive. This discovery is described at greater length in Scholl et al U.S. Pat. Nos. 4,059,466 and 4,059,714, both issued Nov. 22, 1977, entitled "Hot Melt Thermoplastic Adhesive Foam System", and assigned to the assignee of this application, the disclosures of which are hereby incorporated by reference in this application. As shown in those patents, the higher bonding strength of a hot melt adhesive, when foamed, results at least in part from the fact that the foam can be spread over a larger area, under the same compressive conditions, than an equal weight of the same adhesive which has not been foamed. The foam has also been found to have a longer "open" time, after it has been deposited onto a first substrate and during which it can effectively bond to a second substrate when pressed against the latter, yet it has a shorter "tack" time, i.e., it will set up and adhere faster after it has been compressed between two substrates. These characteristics are advantageous in many applications where hot melt adhesives are used.
Hot melt adhesive foams can be produced by mixing a gas such as air or nitrogen, under pressure, with molten hot melt adhesive. when the liquid adhesive/gas mixture is subsequently dispensed, as from a conventional valved type of adhesive dispenser or gun, the gas forms small bubbles throughout the mass, causing the adhesive to expand volumetrically as a foam. If the foam were left in an uncompassed state, it would set up with the air or other gas cells distributed throughout it. However, if the foam is pressed between two substrates before it has cooled, a substantial proportion of the bubbles are crushed, the gas is essentially dispelled from the adhesive, and the adhesive provides the advantageous characteristics mentioned above.